Some of the work vehicles, such as bulldozers, include a hydrostatic transmission (also hereinafter referred to as “HST”). The hydrostatic transmission includes a variable displacement pump and a variable displacement hydraulic motor. The variable displacement pump is configured to be driven by an engine, whereas the variable displacement hydraulic motor is configured to be rotated by the pressurized oil from the variable displacement pump. Further, the torque absorbable by the hydrostatic transmission can be changed or the travel speed of the vehicle can be continuously changed through the displacement control by changing the angle of a swash plate in the variable displacement pump or the variable displacement hydraulic motor.
Further, the work vehicles such as bulldozers are configured to be operated while the engine speed is set to be either the high idle engine speed or a given engine speed less than the high idle engine speed by an operation of the fuel regulation lever/dial (see, e.g., Japan Laid-open Patent Application Publication No. JP-A-2005-233420). In executing a heavy load operation, for instance, the high idle engine speed is selected and the engine is thus rotated at the maximum rated engine speed. In executing a light load operation or self-propelling, by contrast, the vehicles are operated at a low engine speed set by an operation of the fuel regulation lever. It is thereby possible to execute reduction in noises to be produced in operations, reduction in fuel consumption, and the like.
By the way, reduction in fuel consumption has been demanded for the work vehicles such as bulldozers in recent years.
Japan Laid-open Patent Application Publication No. JP-A-S563-070311 discusses a technology aimed at reduction in fuel consumption for a construction vehicle. However, the construction vehicle is not the hydrostatic transmission vehicle intended for the present invention, and therefore it is not necessarily valid to simply compare them. In the technology described in Japan Laid-open Patent Application Publication No. JP-A-S63-070311, a regulation line to determine the maximum engine speed is shifted to a line corresponding to an engine speed less than the maximum rated engine speed when the gear stage of the transmission is detected as the maximum gear stage. Japan Laid-open Patent Application Publication No. JP-A-S63-070311 describes that the fuel consumption rate is thus enhanced when the maximum gear stage is selected.